Transcript 10-DesignPatterns - Department of Computer Science

CSC 335: Object-Oriented
Programming and Design
Object-Oriented
Another question to total 150
Design Patterns
points
Outline
Overview of Design Patterns
Four Design Patterns
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Iterator
Decorator
Strategy
Observer
The Beginning of Patterns
Christopher Alexander, architect
– A Pattern Language--Towns, Buildings, Construction
– Timeless Way of Building (1979)
– “Each pattern describes a problem which occurs over
and over again in our environment, and then describes
the core of the solution to that problem, in such a way
that you can use this solution a million times over,
without ever doing it the same way twice.”
Other patterns: novels (tragic, romantic, crime),
movies genres (drama, comedy, documentary)
“Gang of Four” (GoF) Book
Design Patterns: Elements of Reusable Object-Oriented
Software, Addison-Wesley Publishing Company, 1994
Written by this "gang of four"
– Dr. Erich Gamma, then Software Engineer, Taligent, Inc.
– Dr. Richard Helm, then Senior Technology Consultant, DMR Group
– Dr. Ralph Johnson, then and now at University of Illinois, Computer
Science Department
– Dr. John Vlissides, then a researcher at IBM
• Thomas J. Watson Research Center
• See John's WikiWiki tribute page http://c2.com/cgi/wiki?JohnVlissides
Object-Oriented Design Patterns
This book defined 23 patterns in three categories
– Creational patterns deal with the process of object creation
– Structural patterns, deal primarily with the static composition and
structure of classes and objects
– Behavioral patterns, which deal primarily with dynamic interaction
among classes and objects
Documenting Discovered Patterns
Many other patterns have been introduced
documented
– For example, the book Data Access Patterns by Clifton
Nock introduces 4 decoupling patterns, 5 resource patterns,
5 I/O patterns, 7 cache patterns, and 4 concurrency patterns.
– Other pattern languages include telecommunications
patterns, pedagogical patterns, analysis patterns
– Patterns are mined at places like Patterns Conferences
ChiliPLoP
Recent patterns books work shopped at
ChiliPLoP, Wickenburg and Carefree Arizona
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Patterns of Enterprise Application Arhitecture Martin Fowler
Patterns of Fault Tolerant Software, Bob Hamner
Patterns in XML Fabio Arciniegas
Patterns of Adopting Agile Development Practices Amr
Elssamadisy
– 2010: Patterns of Parallel Programming, Ralph Johnson
• 16 patterns and one Pattern Language work shopped
GoF Patterns
– Creational Patterns
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Abstract Factory
Builder
Factory Method
Prototype
Singleton
– Structural Patterns
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Adapter
Bridge
Composite
Decorator
Façade
Flyweight
Proxy
– Behavioral Patterns
• Chain of Responsibility
• Command
• Interpreter
• Iterator
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Mediator
Memento
Observer
State
• Strategy
• Template Method
• Visitor
Why Study Patterns?
Reuse tried, proven solutions
– Provides a head start
– Avoids gotchas later (unanticipated things)
– No need to reinvent the wheel
Establish common terminology
– Design patterns provide a common point of reference
– Easier to say, “We could use Strategy here.”
Provide a higher level prospective
– Frees us from dealing with the details too early
Other advantages
Most design patterns make software more
modifiable, less brittle
– we are using time tested solutions
Using design patterns makes software systems
easier to change—more maintainable
Helps increase the understanding of basic objectoriented design principles
– encapsulation, inheritance, interfaces, polymorphism
Style for Describing Patterns
We will use this structure:
– Pattern name
– Recurring problem: what problem the pattern
addresses
– Solution: the general approach of the pattern
– UML for the pattern
• Participants: a description as a class diagram
– Use Example(s): examples of this pattern, in Java
A few OO Design Patterns
Coming up:
– Iterator
• access the elements of an aggregate object
sequentially without exposing its underlying
representation
– Strategy
• A means to define a family of algorithms, encapsulate
each one as an object, and make them interchangeable
– Observer a preview
• One object stores a list of observers that are updated
when the state of the object is changed
Iterator
Pattern: Iterator
Name: Iterator (a.k.a Enumeration)
Recurring Problem: How can you loop over all objects
in any collection. You don’t want to change client code
when the collection changes. Want the same methods
Solution: 1) Have each class implement an interface,
and 2) Have an interface that works with all collections
Consequences: Can change collection class details
without changing code to traverse the collection
GoF Version
of Iterator page 257
ListIterator
First()
Next()
IsDone()
CurrentItem()
// A C++ Implementation
ListIterator<Employee> itr = list.iterator();
for(itr.First(); !itr.IsDone(); itr.Next()) {
cout << itr.CurrentItem().toString();
Java version of Iterator
interface Iterator
boolean hasNext()
Returns true if the iteration has more elements.
Object next()
Returns the next element in the iteration and updates the iteration to
refer to the next (or have hasNext() return false)
void remove()
Removes the most recently visited element
Java’s Iterator interface
// The Client code
List<BankAccount> bank =
new ArrayList<BankAccount>();
bank.add(new BankAccount("One", 0.01) );
// ...
bank.add(new BankAccount("Nine thousand", 9000.00));
String ID = "Two";
Iterator<BankAccount> itr = bank.iterator();
while(itr.hasNext()) {
if(itr.next().getID().equals(searchAcct.getID()))
System.out.println("Found " + ref.getID());
}
UML Diagram of Java's
Iterator with a few Collections
<<interface>>
<<interface>>
List
iterator(): Iterator
…
Vector
iterator()
LinkedList
iterator()
Iterator
hasNext()
next()
ArrayList
iterator()
Client
http://download.oracle.com/javase/6/docs/api/java/util/List.html
Iterator
hasNext()
next()
Decorator Design Pattern
Rick Mercer
CSC 335: Object-Oriented
Programming and Design
The Decorator Pattern
from GoF
Intent
– Attach additional responsibilities to an object dynamically.
Decorators provide a flexible alternative to sub classing to
extend flexibility
Also Known As Wrapper
Motivation
– Want to add properties to an existing object.
2 Examples
• Add borders or scrollbars to a GUI component
• Add stream functionality such as reading a line of input or
compressing a file before sending it over the wire
Applicability
Use Decorator
– To add responsibilities to individual objects
dynamically without affecting other objects
– When extending classes is impractical
• Sometimes a large number of independent extensions
are possible and would produce an explosion of
subclasses to support every combination (this
inheritance approach is on the next few slides)
An Application
Suppose there is a TextView GUI component
and you want to add different kinds of borders
and/or scrollbars to it
You can add 3 types of borders
– Plain, 3D, Fancy
and 1 or 2 two scrollbars
– Horizontal and Vertical
An inheritance solution requires15 classes for
one view
That’s a lot of classes!
1.TextView_Plain
2.TextView_Fancy
3.TextView_3D
4.TextView_Horizontal
5.TextView_Vertical
6.TextView_Horizontal_Vertical
7.TextView_Plain_Horizontal
8.TextView_Plain_Vertical
9.TextView_Plain_Horizontal_Vertical
10.TextView_3D_Horizontal
11.TextView_3D_Vertical
12.TextView_3D_Horizontal_Vertical
13.TextView_Fancy_Horizontal
14.TextView_Fancy_Vertical
15.TextView_Fancy_Horizontal_Vertical
Disadvantages
Inheritance solution has an explosion of classes
If another view were added such as StreamedVideoView,
double the number of Borders/Scrollbar classes
Solution to this explosion of classes?
– Use the Decorator Pattern instead
VisualComponent
draw()
resize()
TextView
draw()
resize()
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SteamedVideoView
draw()
resize()
Decorator
draw()
resize()
1
Decorator contains a
visual component
An imagined
example
Plain
draw()
resize()
Border
draw()
resize()
3D
draw()
resize()
Fancy
draw()
resize()
ScrollBar
draw()
resize()
Horiz
draw()
resize()
Vert
draw()
resize()
Decorator's General Form
JScrollPane
Any Component such as Container, JList,
Panel can be decorated with a JScrollPane
The next slide shows how to decorate a JPanel
with a JScrollPane
Decorate a JPanel
JScrollPane scrollPane = new JScrollPane(toStringView);
add(scrollPane); // Add to a JFrame or another panel
Motivation Continued
The more flexible containment approach encloses the
component in another object that adds the border
The enclosing object is called the decorator
The decorator conforms to the interface of the
component so its presence is transparent to clients
The decorator forwards requests to the component and
may perform additional actions before or after any
forwarding
Decorator Design: Java Streams
InputStreamReader(InputStream in)
System.in is an InputStream object
– ... bridge from byte streams to character streams: It reads bytes
and translates them into characters using the specified character
encoding. JavaTMAPI
BufferedReader
– Read text from a character-input stream, buffering characters so as
to provide for the efficient reading of characters, arrays, and lines.
JavaTMAPI
What we had to do for console input before Java 1.5’s Scanner
BufferedReader keyboard =
new BufferedReader(new
InputStreamReader(System.in));
Decorator pattern in the real world
BufferedReader decorates InputStreamReader
BufferedReader
readLine() // add a useful method
InputStreamReader
read()
// 1 byte at a time
close()
Still needed to parse integers, doubles, or words
Java streams
With > 60 streams in Java, you can create a wide
variety of input and output streams
– this provides flexibility good
• it also adds complexity
– Flexibility made possible with inheritance and classes
that accept classes that extend the parameter type
Another Decorator Example
We decorated a FileInputStream with an
ObjectInputStream to read objects that
implement Serializable
– and we used FileOutputStream with
ObjectOutputStream
– then we were able to use nice methods like these two
read and write large complex objects on the file system:
\
outFile.writeObject(list);
// and later on …
list = (ArrayList<String>)inFile.readObject();
Another Decorator Example
Read a plain text file and compress it using the
GZIP format ZIP.java
Read a compress file in the GZIP format and write
it to a plain text file UNGZIP.java
Sample text iliad10.txt from Project Gutenberg
bytes
875,736 iliad10.txt bytes
305,152 iliad10.gz
875,736 TheIliadByHomer
(after code on next slide)
// Open the input file
String inFilename = "iliad10.txt";
FileInputStream input = new FileInputStream(inFilename);
// Open the output file
String outFilename = "iliad10.gz";
GZIPOutputStream out = new GZIPOutputStream(
new FileOutputStream(outFilename));
// Transfer bytes from output file to compressed file
byte[] buf = new byte[1024];
int len;
while ((len = input.read(buf)) > 0) {
out.write(buf, 0, len);
}
// Close the file and stream
input.close();
out.close();
// Open the gzip file
String inFilename = "iliad10.gz";
GZIPInputStream gzipInputStream =
new GZIPInputStream(new FileInputStream(inFilename));
// Open the output file
String outFilename = "TheIliadByHomer";
OutputStream out = new FileOutputStream(outFilename);
// Transfer bytes from compressed file to output file
byte[] buf = new byte[1024];
int len;
while ((len = gzipInputStream.read(buf)) > 0) {
out.write(buf, 0, len);
}
// Close the file and stream
gzipInputStream.close();
out.close();
GZIPInputStream is a Decorator
GZIPInputStream
Summary
Decorators are very flexible alternative of
inheritance
Decorators enhance (or in some cases restrict)
the functionality of decorated objects
They work dynamically to extend class
responsibilities, even inheritance does some but
in a static fashion at compile time
Strategy Design Pattern
Strategy
Pattern: Strategy
Name: Strategy (a.k.a Policy)
Problem: You want to encapsulate a family of
algorithms and make them interchangeable.
Strategy lets the algorithm vary independently
from the clients that use it (GoF)
Solution: Create an abstract strategy class (or
interface) and extend (or implement) it in
numerous ways. Each subclass defines the
same method names in different ways
Design Pattern: Strategy
Consequences:
– Allows families of algorithms
Known uses:
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Critters seen in section for Rick’s 127B / 227
Layout managers in Java
Different Poker Strategies in a 335 Project
Different PacMan chase strategies in a 335 Project
Different Jukebox policies that can be
Java Example of Strategy
this.setLayout(new FlowLayout());
this.setLayout(new GridLayout());
In Java, a container HAS-A layout manager
– There is a default
– You can change a container's layout manager with
a setLayout message
Change the stategy at runtime
Demonstrate
LayoutControllerFrame.java
private class FlowListener
implements ActionListener {
// There is another ActionListener for GridLayout
public void actionPerformed(ActionEvent evt) {
// Change the layout strategy of the JPanel
// and tell it to lay itself out
centerPanel.setLayout(new FlowLayout());
centerPanel.validate();
}
}
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interface LayoutManager
– Java has interface java.awt.LayoutManager
– Known Implementing Classes
• GridLayout, FlowLayout, ScrollPaneLayout
– Each class implements the following methods
addLayoutComponent(String name, Component comp)
layoutContainer(Container parent)
minimumLayoutSize(Container parent)
preferredLayoutSize(Container parent)
removeLayoutComponent(Component comp)
UML Diagram of Strategy
General Form
Context
strategy: Strategy
<<interface>>
Strategy
AlgorithmInterface
setStrategy(Strategy)
…
implements
ConcreteClassA
AlgorithmInterface
ConcreteClassB
AlgorithmInterface
ConcreteClassC
AlgorithmInterface
Specific UML Diagram of
LayoutManager in Java
<<interface>>
JPanel
layoutMan: LayoutManager
size: Dimension
setLayout(lm: LayoutManager)
setPreferredSize(di:Dimension)
LayoutManager
addLayoutComponent()
layoutContainer()
minimumLayoutSize()
implements
GridLayout
FlowLayout
ScrollPaneLayout
addLayoutComponent()
layoutContainer()
minimumLayoutSize()
addLayoutComponent()
layoutContainer()
minimumLayoutSize()
addLayoutComponent()
layoutContainer()
minimumLayoutSize()
Another Example
– Pac Man GhostChasesPacMan strategies in 2001
– Level 1: random
– Level 2: a bit smarter
– Level 3: use a shortest path algorithm
http://www.martystepp.com/applets/pacman/
– Could be interface ChaseStategy is in the Ghost class
interface ChaseStategy {
public Point nextPointToMoveTo();
}
– and Ghost has setChaseStrategy(new ShortestPath())
The Observer Design Pattern
Name: Observer
Problem: Need to notify a changing number of
objects that something has changed
Solution: Define a one-to-many dependency
between objects so that when one object
changes state, all its dependents are notified
and updated automatically
Examples
From Heads-First: Send a newspaper to all who
subscribe
– People add and drop subscriptions, when a new
version comes out, it goes to all currently described
Spreadsheet
– Demo: Draw two charts—two views--with some
changing numbers--the model
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Examples
File Explorer (or Finders) are registered
observers (the view) of the file system (the
model).
Demo: Open several finders to view file system
and delete a file
Later in Java: We'll have two views of the same
model that get an update message whenever the
state of the model has changed
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Observer Example